A number of nontricyclic antidepressants have recently been developed that diminish the cardiovascular and anticholinergic liability characteristic of tricyclic antidepressants. Some of these compounds are used as anti-obesity agents and have shown promise in the treatment of cerebral function disorders such as Parkinson's disease and senile dementia. See, e.g., WO 94/00047 and WO 94/00114. The nontricyclic compound, venlafaxine, chemically named (±)-1-[2-(dimethylamino)-1-(4-methoxyphenyl)ethyl]-cyclohexanol, is an antidepressant which has been studied extensively and which is described in, for example, U.S. Pat. No. 4,761,501 and Pento, J. T. Drugs of the Future 13(9):839-840 (1988). Its hydrochloride salt is currently commercially available in the United States under the trade name Effexor®. Effexor®, which is a racemic mixture of the (+) and (−) enantiomers of venlafaxine, is indicated for the treatment of depression.
Although venlafaxine contains an asymmetric carbon atom and is sold as a racemate, it has been reported that its (−) enantiomer is a more potent inhibitor of norepinephrine synaptosomal uptake while its (+) enantiomer is more selective in inhibiting serotonin uptake. Howell, S. R. et al. Xenobiotica 24(4):315-327 (1994). Furthermore, studies have shown that the ratio of the two isomers' metabolism varies not only among species, but between subjects as well. Klamerus, K. J. et al. J. Clin. Pharmacol. 32:716-724 (1992). In humans, venlafaxine is transformed by a saturable metabolic pathway into two minor metabolites, N-desmethylvenlafaxine and N,O-didesmethylvenlafaxine, and one major metabolite, O-desmethylvenlafaxine, as shown in Scheme I(a):
Klamerus, K. J. et al. J. Clin. Pharmacol. 32:716-724 (1992). All of these metabolites are racemic. In vitro studies suggest that O-desmethylvenlafaxine is a more potent inhibitor of norepinephrine and dopamine uptake than the parent compound racemic venlafaxine. Muth, F. A. et al. Drug Develop. Res. 23:191-199 (1991). O-desmethylvenlafaxine has also been reported to have a half-life (t½) of about 10 hours, which is approximately 2.5 times as long as that of venlafaxine. Klamerus, K. J. et al. J. Clin. Pharmacol. 32:716-724 (1992). Studies directed at understanding the activity of O-desmethylvenlafaxine as compared to its parent have been hampered, however, by the metabolic difference between laboratory animals and man in their exposure to venlafaxine. Howell, S. R. et al. Xenobiotica 24(4):315-327 (1994).
Despite the benefits of racemic venlafaxine, it has adverse effects including, but not limited to, sustained hypertension, headache, asthenia, sweating, nausea, constipation, somnolence, dry mouth, dizziness, insomnia, nervousness, anxiety, blurred or blurry vision, and abnormal ejaculation/orgasm or impotence in males. Physicians' Desk Reference pp. 3293-3302 (53rd ed., 1999); see also Sinclair, J. et al. Rev. Contemp. Pharmacother. 9:333-344 (1998). These adverse effects can significantly limit the dose level, frequency, and duration of drug therapy. It would thus be desirable to find a compound with the advantages of venlafaxine while avoiding its disadvantages.